This invention relates to ink jet printing method and apparatus. More specifically, the invention relates to a fluid drop generation method and apparatus of the type wherein drops are generated from a continuous stream of liquid emitted under pressure through a nozzle.
The present type of continuous drop ink jet system is described in U.S. Pat. No. 3,596,275 issued on July 27, 1971 to Richard G. Sweet. The Sweet patent describes three techniques for stimulating or exciting the fluid to obtain a substantially fixed generation rate of drops of equal size and spacing at a stable distance from the nozzle. Among them is a movable member or diaphragm driven by a magnetostrictive or piezoelectric driver located outside the cavity containing the ink. A vibrating nozzle and electrohydrodynamic excitor are the other two types of excitors disclosed by Sweet.
Another piezoelectric device is disclosed in U.S. Pat. No. 3,900,162 to Titus and Tsao wherein a piezoelectric strip bonded to a stainless steel sheet divides a diamond shaped ink cavity into two compartments. The stainless steel sheet is substituted for the diaphragm in Sweet. Another bending diaphragm is disclosed by Denny, Loeffler and West in the August, 1973 issue of the IBM Technical Disclosure Bulletin at pages 789-91, Vol. 16, No. 3. There the bending device is referred to as a bimorphic-piezoelectric ceramic crystal.
U.S. Pat. No. 4,138,687 to Cha and Hou, employs another variation of the movable diaphragm. This patent discloses a pair of piezoelectric ceramic devices sandwiched between two rigid blocks, one called a backing plate and the other a piston. The piston extends into the fluid reservoir and as it is forced up and down by the ceramic transducers it acts upon the printing liquid to form plane waves that propogate through the liquid toward orifices opposite the piston. The entire transducer is coupled to the reservoir block by a holder that isolates the vibration of the transducer from the reservoir block. See also disclosure number 18010 at page 140 of the April 1979 edition of Research Disclosure wherein the piston is mercury.
The above and like transducers share a common trait in that each uses a vibrating diaphragm as one wall of the fluid reservoir. The piezoelectric transducer is designed to have an acoustic resonant frequency matched to a desired drop generation frequency. Design problems are especially troublesome in generators that create multiple parallel streams of fluid drops. Prior piezoelectrics transducers used in ink jet application are limited in acoustic bandwidth thereby necessitating that the geometry of the reservoir be tailored to a resonant frequency compatible with the transducer. This need to match to the driver resonance to a desired drop generation frequency inhibits design freedom for various ink jet applications.